This invention relates generally to centrifugal pumps, and, more particularly, to an improved method and apparatus for determining degradation of a centrifugal pump.
As is known, a centrifugal pump has a wheel fitted with vanes and known as an impeller. The impeller imparts motion to the fluid which is directed through the pump. A centrifugal pump provides a relatively steady fluid flow. The pressure for achieving the required head is produced by centrifugal acceleration of the fluid in the rotating impeller. The fluid flows axially towards the impeller, is deflected by it and flows out through apertures between the vanes. Thus, the fluid undergoes a change in direction and is accelerated. This produces an increase in the pressure at the pump outlet. When leaving the impeller, the fluid may first pass through a ring of fixed vanes which surround the impeller and is commonly referred to as a diffuser. In this device, with gradually widening passages, the velocity of the liquid is reduced, its kinetic energy being converted into pressure energy. Of course it is noted that in some centrifugal pumps there is no diffuser and the fluid passes directly from the impeller to the volute. The volute is a gradual widening of the spiral casing of the pump. Centrifugal pumps are well known and are widely used in many different environments and applications.
The prior art also refers to centrifugal pumps as velocity machines because the pumping action requires first, the production of the liquid velocity; second, the conversion of the velocity head to a pressure head. The velocity is given by the rotating impeller, the conversion accomplished by diffusing guide vanes in the turbine type and in the volute case surrounding the impeller in the volute type pump. With a few exceptions, all single stage pumps are normally of the volute type. Specific speed Ns of the centrifugal pump is NQ1/2/H3/4. Ordinarily, N is expressed in rotations per minute, Q in gallons per minute and head (H) in feet. The specific speed of an impeller is an index to its type. Impellers for high heads usually have low specific speeds, while those for low heads have high specific speeds. The specific speed is a valuable index in determining the maximum suction head that may be employed without the danger of cavitation or vibration, both of which adversely effect capacity and efficiency. Operating points of centrifugal pumps are extremely important.
Several common methods are employed in the prior art to monitor and detect when the centrifugal pump""s performance degrades. One such technique operates on the fixed speed pump. The flow and total dynamic head (TDH) is measured when the pump is new. This information is stored as a graph, table or polynomial curve. As the pump ages, the flow and TDH are measured periodically and compared to the new flow and TDH. If the TDH at a given flow drops below a preset percentage, the pump has degraded to a level whereby the pump would have to be either replaced or rebuilt.
A second technique operates on a fixed speed pump. The flow and brake horsepower (BHP) is measured when the pump is new. The information is again stored as a graph, table or polynomial curve. As the pump ages, the flow and BHP are measured periodically and compared to the original flow and BHP. If the BHP at a given flow and the same speed has increased above a preset percentage, the pump and/or motor have degraded to a level that further investigation is needed to determine which rotating piece of equipment is in need on being rebuilt or whether a new pump is necessary. This works well on pumpages whose specific gravity or viscosity does not change in time.
In the third instance, on a variable speed pump, the flow and TDH are measured at several speeds when the pump is new. This information is again stored in a series of graphs, tables or polynomial curves. As the pump ages, the speed, flow and TDH are measured periodically and compared to the original flow and TDH using the Affinity Law to convert the measurements to the nearest speed curve. If the TDH at a given flow drops below a present percentage, the pump has degraded to an undesirable level. This level would indicate that a rebuilt pump is required or that the pump should be replaced.
In regard to the above, it is seen that certain of the methods require that four separate sensing devices (transducers) be purchased and permanently installed on the pump. These devices are to measure suction pressure, discharge pressure, temperature and flow. Therefore, as one can ascertain, the pressure measuring devices are typical pressure transducers, while temperature devices may be temperature sensitive elements, such as thermistors and so on, and flow measuring devices are also well known. The capital expenditures involved in installing and maintaining these sensors are expensive and substantially increase the cost of the unit. In the second method, determining the fixed speed pump operation, that requires the same four transducers or sensor devices utilized to calculate the TDH, but also requires that the pumpage does not change specific gravity or viscosity to cause a change in BHP which is not associated with pump wear.
Thus, as one can ascertain, the prior art techniques are expensive and require the use of additional sensing devices which are permanently installed and become part of the pump.
It is therefore an object of the present invention to provide an improved method and apparatus for detecting degradation performance of a centrifugal pump without employing excessive additional transducer devices.
The present invention requires the use of a variable speed drive (VSD) for the pump motor. The drive utilized has the ability to characterize the motor to obtain torque supplied by the motor and the actual motor running speed. This feature is provided in most variable frequency drives as presently implemented in today""s technology. There is one additional pump sensor required. This sensor measures the differential pressure across the pump, pump discharge pressure or flow. The single additional sensor is also permanently installed with the pump. The apparatus and method clearly requires only one pump transmitter or transducer as opposed to the four required by prior art systems.